Micromechanical spatial light modulators (SLM) have recently been developed. One type of SLM has been termed "deformable mirror device" (DMD). However, the mirror device is not actually deformed. Instead, the mirror device is formed in a hinged architecture so that a reflective surface or mirror is tilted. Alternatively, a reflective surface of a mirror is formed on a torsion beam which is selectively rotated.
DMD modulators are monolithally integrated, silicon based devices manufactured with conventional CMOS processes using standard materials. The modulators consist of a single row or multiple rows of pixel mirrors, each pixel mirror being independently addressable.
U.S. Pat. No. 5,159,485 to Nelson discloses an optical system for illuminating a DMD array. Nelson uses a modified Kohler type illumination system with an anamorphic element in order to attempt to create uniform illumination. The Kohler optics is widely used in projection systems in transmission mode, where the film is placed in the converging light cone between the condenser lens and the projection lens in such a fashion that while the film is in the object plane of the projection lens, the condenser images the light source into the aperture of the (same) projection lens. While the Kohler system provides reasonable performance in a conventional film projection system, it suffers from the so called "cosine 4th" losses ( non-uniform illumination), and the application proposed by Nelson creates additional problems.
There are two basic requirements for acceptable digital image creation in a xerographic system: sufficient energy level and proper spot size. The former is a function of the power of the light source and the throughput efficiency of the optical system. The latter is controlled by the F/number, which in turn is the function of the "aperture size" and the focal length of the imaging optics ( diffraction limited optics assumed). Here the "aperture size" means the actually illuminated region of the physical lens opening.
In a conventional Kohler system, the size of the film ( negative or positive) is such that if it is placed in the light cone emerging from the condenser, it covers a very large part of it, thus most of the light energy will arrive in the aperture of the projection lens. Although the aperture will not necessarily be completely filled, the resolution of the image will be acceptable because these optical systems are rarely if ever diffraction limited.
In the system proposed by Nelson, the small ( aprox. 15 to 30 micron) pixels mirrors are acting as light path folding components in a converging cone. Therefore, as they direct the already small cross section of beam into an even smaller image in the aperture of the projection lens, the effective aperture of the projection lens will be too small to provide the F/# that is required for the desired spot size for high resolution printing.